| From: |
| Puranjay Mohan <puranjay-AT-kernel.org> |
| To: |
| bpf-AT-vger.kernel.org |
| Subject: |
| [PATCH bpf-next 0/4] bpf: verifier: Improve state pruning for scalar registers |
| Date: |
| Mon, 02 Feb 2026 02:44:02 -0800 |
| Message-ID: |
| <20260202104414.3103323-1-puranjay@kernel.org> |
| Cc: |
| Puranjay Mohan <puranjay-AT-kernel.org>, Puranjay Mohan <puranjay12-AT-gmail.com>, Alexei Starovoitov <ast-AT-kernel.org>, Andrii Nakryiko <andrii-AT-kernel.org>, Daniel Borkmann <daniel-AT-iogearbox.net>, Martin KaFai Lau <martin.lau-AT-kernel.org>, Eduard Zingerman <eddyz87-AT-gmail.com>, Kumar Kartikeya Dwivedi <memxor-AT-gmail.com>, Mykyta Yatsenko <mykyta.yatsenko5-AT-gmail.com>, kernel-team-AT-meta.com, Andrii Nakryiko <andrii.nakryiko-AT-gmail.com> |
| Archive-link: |
| Article |
This series improves BPF verifier state pruning by relaxing scalar ID
equivalence requirements. Scalar register IDs are used to track
relationships between registers for bounds propagation. However, once
an ID becomes "singular" (only one register/stack slot carries it), it
can no longer participate in bounds propagation and becomes stale.
These stale IDs can prevent pruning of otherwise equivalent states.
The series addresses this in four patches:
Patch 1: Assign IDs on stack fills to ensure stack slots have IDs
before being read into registers, preparing for the singular ID
clearing in patch 2.
Patch 2: Clear IDs that appear only once before caching, as they cannot
contribute to bounds propagation.
Patch 3: Relax maybe_widen_reg() to only compare value-tracking fields
(bounds, tnum, var_off) rather than also requiring ID matches. Two
scalars with identical value constraints but different IDs represent
the same abstract value and don't need widening.
Patch 4: Relax scalar ID equivalence in state comparison by treating
rold->id == 0 as "independent". If the old state didn't rely on ID
relationships for a register, any linking in the current state only
adds constraints and is safe to accept for pruning.
I ran veristat on BPF programs from sched_ext, meta's internal programs,
and on selftest programs, showing programs with insn diff > 5%:
Scx Progs
File Program States (A) States (B) States (DIFF) Insns (A) Insns (B) Insns (DIFF)
------------------ ------------------- ---------- ---------- ------------- --------- --------- ---------------
scx_rusty.bpf.o rusty_set_cpumask 320 230 -90 (-28.12%) 4478 3259 -1219 (-27.22%)
scx_bpfland.bpf.o bpfland_select_cpu 55 49 -6 (-10.91%) 691 618 -73 (-10.56%)
scx_beerland.bpf.o beerland_select_cpu 27 25 -2 (-7.41%) 320 295 -25 (-7.81%)
scx_p2dq.bpf.o p2dq_init 265 250 -15 (-5.66%) 3423 3233 -190 (-5.55%)
scx_layered.bpf.o layered_enqueue 1461 1386 -75 (-5.13%) 14541 13792 -749 (-5.15%)
FB Progs
File Program States (A) States (B) States (DIFF) Insns (A) Insns (B) Insns (DIFF)
------------ ------------------- ---------- ---------- -------------- --------- --------- ---------------
bpf007.bpf.o bpfj_free 1726 1342 -384 (-22.25%) 25671 19096 -6575 (-25.61%)
bpf041.bpf.o armr_net_block_init 22373 20411 -1962 (-8.77%) 651697 602873 -48824 (-7.49%)
bpf227.bpf.o layered_quiescent 28 26 -2 (-7.14%) 365 340 -25 (-6.85%)
bpf248.bpf.o p2dq_init 263 248 -15 (-5.70%) 3370 3159 -211 (-6.26%)
bpf254.bpf.o p2dq_init 263 248 -15 (-5.70%) 3388 3177 -211 (-6.23%)
bpf241.bpf.o p2dq_init 264 249 -15 (-5.68%) 3428 3240 -188 (-5.48%)
bpf230.bpf.o p2dq_init 287 271 -16 (-5.57%) 3666 3431 -235 (-6.41%)
bpf251.bpf.o lavd_cpu_offline 321 316 -5 (-1.56%) 6221 5891 -330 (-5.30%)
bpf251.bpf.o lavd_cpu_online 321 316 -5 (-1.56%) 6219 5889 -330 (-5.31%)
Selftest Progs
File Program States (A) States (B) States (DIFF) Insns (A) Insns (B) Insns (DIFF)
---------------------------------- ----------------- ---------- ---------- ------------- --------- --------- ---------------
verifier_iterating_callbacks.bpf.o test2 4 2 -2 (-50.00%) 29 18 -11 (-37.93%)
verifier_iterating_callbacks.bpf.o test3 4 2 -2 (-50.00%) 31 19 -12 (-38.71%)
strobemeta_bpf_loop.bpf.o on_event 318 221 -97 (-30.50%) 3938 2755 -1183 (-30.04%)
bpf_qdisc_fq.bpf.o bpf_fq_dequeue 133 105 -28 (-21.05%) 1686 1385 -301 (-17.85%)
iters.bpf.o delayed_read_mark 6 5 -1 (-16.67%) 60 46 -14 (-23.33%)
arena_strsearch.bpf.o arena_strsearch 107 106 -1 (-0.93%) 1394 1258 -136 (-9.76%)
Puranjay Mohan (4):
bpf: verifier: Assign ids on stack fills
bpf: verifier: Clear singular ids for scalars
bpf: verifier: relax maybe_widen_reg() constraints
bpf: verifier: Relax scalar id equivalence for state pruning
include/linux/bpf_verifier.h | 7 +-
kernel/bpf/verifier.c | 169 ++++++++++++++++--
.../selftests/bpf/progs/verifier_scalar_ids.c | 8 +-
3 files changed, 160 insertions(+), 24 deletions(-)
base-commit: 4bebb99140c7b6ee6d894c40557f938804fa8693
--
2.47.3
